Specifies that the type is a pointer-like object which can be compared to std::nullptr_t
objects.
The type must meet all of the following requirements:
In addition, a value-initialized object of the type must produce a null value of that type. This null value shall only be equivalent to itself. Default initialization of the type may have an indeterminate value.
An object of the type must be contextually convertible to bool. The effect of this conversion returns false if the value is equivalent to its null value and true otherwise.
None of the operations that this type performs may throw exceptions.
The type must satisfy the following additional expressions, given two values p
and q
that are of the type, and that np
is a value of std::nullptr_t
type (possibly const
qualified):
Expression | Effects |
---|---|
Type p(np);
| Afterwards, p is equivalent to nullptr |
Type(np) | a temporary object that is equivalent to nullptr |
p = np | Must return a Type& , and afterwards, p is equivalent to nullptr |
p != q | Must return a value that is contextually convertible to bool. The effect is |
p == np
| Must return a value that is contextually convertible to bool. The effect is |
p != np
| Must return a value that is contextually convertible to bool. The effect is |
Note that dereferencing (operator*
or operator->
) is not required of a NullablePointer type. A minimalistic type that satisfies these requirements is.
class handle { int id; public: handle(std::nullptr_t = nullptr) : id(0) { } explicit operator bool() { return id != 0; } friend bool operator ==(handle l, handle r) { return l.id == r.id; } friend bool operator !=(handle l, handle r) { return !(l == r); } };
The following types must satisfy NullablePointer:
X::pointer
, X::const_pointer
, X::void_pointer
and X::const_void_pointer
of every Allocator type X
X::pointer
of std::unique_ptr
std::exception_ptr
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